Method of designing compressible particles having buoyancy in a confined volume

ABSTRACT

A method of designing compressible particles for a fluid mixture. The compressible particles are intended to be used for attenuating pressure within a confined volume such as a trapped annulus. Preferably, the compressible particles reside buoyantly within an aqueous fluid, forming a fluid mixture. Each of the compressible particles is fabricated to collapse in response to fluid pressure within the confined volume, and comprises carbon. The particles may each have a porosity of between 5% and 40%, and a compressibility of between 10% and 30%, at 10,000 psi. The particles are tuned to have a buoyancy that is lower than the carrier fluid while still having resiliency.

CROSS-REFERENCE TO RELATED APPLICATION

This application claims the benefit of U.S. Provisional Application62/758,843 filed Nov. 12, 2018 entitled “Method of DesigningCompressible Particles Having Buoyancy In A Confined Volume,” theentirety of which is incorporated by reference herein. This applicationis technically related to U.S. Provisional Application 62/758,846 filedNov. 12, 2018 entitled “Buoyant Particles Designed For Compressibility,”the entirety of which is incorporated by reference herein. Thisapplication is also technically related to U.S. Provisional Application62/758,852 filed Nov. 12, 2018 entitled, “Method Of Placing A FluidMixture Containing Compressible Particles Into A Wellbore,” the entiretyof which is incorporated by reference herein. This application is alsotechnically related to U.S. Provisional Application 62/758,858 filedNov. 12, 2018 entitled, “A Fluid Mixture Containing CompressibleParticles,” the entirety of which is incorporated by reference herein.This application is also technically related to U.S. ProvisionalApplication 62/758,862 filed Nov. 12, 2018 entitled, “Tubular BodyContaining Compressible Particles, And Method Of Attenuating AnnularPressure,” the entirety of which is incorporated by reference herein.

BACKGROUND OF THE INVENTION

This section is intended to introduce various aspects of the art, whichmay be associated with exemplary embodiments of the present disclosure.This discussion is believed to assist in providing a framework tofacilitate a better understanding of particular aspects of the presentdisclosure. Accordingly, it should be understood that this sectionshould be read in this light, and not necessarily as admissions of priorart.

FIELD OF THE INVENTION

The present disclosure relates to the field of hydrocarbon recoveryoperations. More specifically, the present invention relates to thedrilling and completion of wells. Further, the invention relates tobuoyant particles that may be placed in a confined volume for thepurpose of absorbing pressure.

TECHNOLOGY IN THE FIELD OF THE INVENTION

In the drilling of oil and gas wells, a wellbore is formed using a drillbit that is urged downwardly at a lower end of a drill string. The drillbit is rotated while force is applied through the drill string andagainst the rock face of the formation being drilled. After drilling toa predetermined depth, the drill string and bit are removed and thewellbore is lined with a string of casing.

In completing a wellbore, it is common for the drilling company to placea series of casing strings having progressively smaller outer diametersinto the wellbore. A first string of casing is placed from the surfaceand down to a first drilled depth. This casing is known as surfacecasing. In the case of offshore operations, this casing may be referredto as a conductor pipe. One of the main functions of the initial stringof casing is to isolate and protect the shallower, fresh water bearingaquifers from contamination by wellbore fluids. Accordingly, this casingstring is almost always cemented entirely back to the surface.

One or more intermediate strings of casing is also run into thewellbore. Each successive pipe string extends to a greater depth thanits predecessor, and has a smaller diameter than its predecessor.

The process of drilling and then cementing progressively smaller stringsof casing is repeated several times until the well has reached totaldepth. A final string of casing, referred to as production casing, isused along the pay zones. In some instances, the final string of casingis a liner, that is, a pipe string that is hung in the wellbore using aliner hanger. Frequently today, the final string of casing is a longpipe string that extends along a horizontal portion (or “leg”) of awellbore.

The process of running a string of casing into the wellbore will form anannular area (or “annulus”) between the pipe and the surroundingborehole. A cementing operation is typically conducted in order to fillor “squeeze” selected annular areas with a column of cement. Thecombination of cement and casing strengthens the wellbore andfacilitates the zonal isolation of certain sections of ahydrocarbon-producing formation (or “pay zones”) behind the casing.

In most wellbore completion jobs today, especially those involving socalled unconventional formations where high-pressure hydraulicoperations are conducted downhole, the surface casing and perhaps thefirst intermediate string of casing are entirely cemented up to thesurface. Hydraulic cements, usually Portland cement, are used to cementthe tubular bodies within the wellbore. However, in some completions,particularly those where overlapping strings of casing extend to thesurface, the operator may choose to leave an extended portion of certainintermediate casing strings without cement. This saves the drillingcompany time and the well operator money. However, this also means thatupon completion an extended section of wellbore will have wellborefluids residing on top of a column of cement up to the well head.

FIG. 1A is a cross-sectional view of a wellbore 100 undergoingcompletion. The wellbore 100 defines a bore 10 that has been drilledfrom an earth surface 105 into a subsurface 110. The wellbore 100 isformed using any known drilling mechanism, but preferably using aland-based rig or an offshore drilling rig on a platform. For deeperhorizontal wells such as the one shown in FIG. 1A, the wellbore may beformed at least in part through the use of a downhole motor and MWDelectronics.

The wellbore 100 is completed with a first string of casing 120,sometimes referred to as surface casing. The wellbore 100 is furthercompleted with a second string of casing 130, typically referred to asan intermediate casing. In deeper wells, that is wells completed below7,500 feet, at least two intermediate strings of casing will be used. InFIG. 1A, a second intermediate string of casing is shown at 140.

The wellbore 100 is finally completed with a string of production casing153. In the view of FIG. 1A, the production casing extends from thesurface 105 down to a subsurface formation, or “pay zone” 115. Thewellbore 100 is completed horizontally, meaning that a horizontal “leg”50 is provided. The leg 50 is formed from the end of the productioncasing 150. The leg 50 includes a heel 153 and a toe 154 along the payzone 115. In this instance, the toe 154 defines the end (or “TD”) of thewellbore 100.

It is observed that the annular region around the surface casing 120 isfilled with cement 125. The cement (or cement matrix) 125 serves toisolate the wellbore from fresh water zones and potentially porousformations around the casing string 120 and near the surface 105.

The annular regions around the intermediate casing strings 130, 140 arealso filled with cement 135, 145. Similarly, the annular region aroundthe production casing 150 is filled with cement 155. However, the cement135, 145, 155 is only placed behind the respective casing strings 130,140, 150 up to the lowest joint of the immediately surrounding casingstring, or cement shoe. Thus, a non-cemented annular area 132 ispreserved above the cement matrix 135; a non-cemented annular area 142is preserved above the cement matrix 145; and a non-cemented annulararea 152 is preserved above the cement matrix 155.

FIG. 1B is a perspective view of the wellbore 100 of FIG. 1A, or atleast the upper half of the wellbore 100. Here, casing strings 120, 130,140 and 150 are again shown. In addition, cement matrices 125, 135, 145and 155 are visible. Finally, non-cemented annular areas 132, 142 and152 are shown.

An annulus can be considered “trapped” if the cement pumping places thetop of cement (or “TOC”) higher than the previous shoe. Alternately, ifthe shoe remains open to the formation (not blocked by the cement)solids may settle out from the annular fluid, effectively plugging upthe bottom of the annulus. In any instance, those of ordinary skill inthe art will understand that the non-cemented annular areas 132, 142,152 are not unfilled to above the TOC; rather, they are left withwellbore fluids therein. Such fluids may include drilling fluids,aqueous acid, and formation gas. When the well is completed, a wellheadis placed over the annular areas 132, 142, 152, sealing these regions.For this reason each may be referred to as a “trapped annulus.”

During the course of producing hydrocarbons, warm production fluids flowthrough a tubing string (not shown) up to the surface. These fluidsraise the temperature inside the wellbore 100, including the fluidsinside the one or more trapped annuli 132, 142, 152. This, in turn, willincrease the pressure within each trapped annulus. (Note that the effectof a trapped annulus is that the fluid in the annulus has no path toescape should the pressure rise.) This pressure can exceed the pressureratings (burst or collapse pressures) of the inner strings of casing.For example, a trapped annulus can lead to pipe collapse or even wellfailure.

Accordingly, a need exists for an improved wellbore design that canabsorb burst pressure as wellbore temperature is increased. Further, aneed exists for a unique carrier fluid mixture having collapsibleparticles capable of absorbing an increase in fluid pressure within atrapped annulus. A need further exists for a method of designingcompressible particles having buoyancy within a confined annular space.

BRIEF SUMMARY OF THE DISCLOSURE

A method of designing compressible particles for a fluid mixture isprovided. The compressible particles are intended to be used forattenuating pressure within a confined volume such as a trapped annulus.Preferably, the compressible particles reside within an aqueous fluid,forming a fluid mixture. The aqueous carrier fluid may be fresh water,brine or an aqueous drilling fluid.

The compressible particles are designed to be buoyant when placed withinthe aqueous carrier fluid. In one aspect, the compressible particles areat a concentration of 5% to 40% of the fluid mixture, by volume.

Each of the compressible particles is fabricated to collapse in responseto fluid pressure within the confined volume. Each of the particlespreferably comprises carbon. At least some of the compressible particlesmay comprise graphite or graphene beads. In one embodiment, each of thecompressible particles comprises a porous graphite carbon (PGC)material. In this instance, an inner core is composed of amorphouscarbon, while an outer shell is composed of graphitic carbon. Both theinner core and the outer shell are porous.

In one embodiment, the method first comprises determining a density ofthe aqueous carrier fluid. The method further includes selecting adensity for the compressible particles. The density of the compressibleparticles should be selected to provide for buoyancy within the carrierfluid.

In one aspect, the compressible particles comprise first compressibleparticles having a first selected density, and second compressibleparticles having a second selected density. The first density is greaterthan the second density.

The method also includes selecting a geometry for the compressibleparticles. In one aspect, the selected geometry is either a circularprofile or an oval profile. In another aspect, the selected geometry ispolygonal. The compressible particles may share the same polygonalgeometry or have geometries that vary among them, in which case thegeometries are designed to prevent packing.

The method additionally includes selecting a compressibility response(or “compressibility”) for the particles. The selected compressibilityresponse is between 10% and 30%, or more preferably between 14% and 27%,inclusive, at up to 10,000 psi.

The method further comprises determining a diameter of weighting agentparticles residing within the annular region. The weighting agentparticles represent particles of barite or hematite or other residualagents from a drilling mud. The method then includes selecting adiameter for at least a portion of the compressible particles that is atleast 100 times greater than the determined diameter for the weightingagent particles, thereby providing enlarged particles.

The method also includes adjusting the density of the compressibleparticles to ensure buoyancy in the presence of the weighting agentparticles. In this respect, once a diameter is provided for the enlargedportion of the particles, density may need to be reduced in order topreserve buoyancy.

In the method, each of the compressible particles may have an outerdiameter that is between 10 μm and 700 μm (in dry state). Morepreferably, the particle size distribution will be between 100 μm to 700μm. Together, the particles may each have a porosity of between 5% and40%, or the particles together may have an average porosity of between10% and 25%.

Each of the particles has a resiliency of between 80% and 120%, and morepreferably between 87% and 117%, inclusive. Preferably, the fluidmixture further comprises a reductant provided to reduce gas content.

In one embodiment, each of the compressible particles has a density thatis between 12.0 ppg and 12.8 ppg, inclusive. The compressible particlesare mixed into the aqueous carrier medium, forming a fluid mixture.

BRIEF DESCRIPTION OF THE DRAWINGS

So that the manner in which the present inventions can be betterunderstood, certain illustrations, charts and/or flow charts areappended hereto. It is to be noted, however, that the drawingsillustrate only selected embodiments of the inventions and are thereforenot to be considered limiting of scope, for the inventions may admit toother equally effective embodiments and applications.

FIG. 1A is a side view of a wellbore. The wellbore has a plurality ofcasing strings cemented into place, and is completed with a string ofproduction casing.

FIG. 1B is a side perspective view of an upper half of the wellbore ofFIG. 1A. Three annular regions are shown as trapped annuli.

FIG. 2A is a perspective view of a tubular body of the presentinvention, in one embodiment. In a preferred aspect, the tubular body isdeployed in a wellbore as part of a string of casing.

FIG. 2B is a perspective, cut-away view of a filter screen placed on thetubular body of FIG. 2A. The filter screen holds a plurality ofcompressible particles.

FIGS. 3A and 3B present side views of filter screens in alternatearrangements. Either filter screen may be fitted around an outer body ofa joint of casing, and at least partially filled with compressibleparticles.

FIG. 3A presents the filter screen of FIG. 2A in an embodiment that issimilar to a sand screen.

FIG. 3B presents the filter screen of FIG. 2A as a slotted tubular.

FIG. 4 is a perspective view of the upper portion of a wellbore havingtrapped annuli. An intermediate string of casing has received a seriesof filter screens, such as either of the filter screens of FIG. 3A or3B.

FIG. 5A is another perspective view of the upper portion of a wellbore,in an alternate embodiment. In this instance, a column of carrier fluidholding compressible particles is placed in the unfilled annular spacearound an intermediate string of casing. The compressible particles aresuspended in and dispersed along the column of carrier fluid.

FIG. 5B is still another perspective view of the upper portion of awellbore. In this instance, a column of carrier fluid holdingcompressible particles is again placed in the unfilled annular spacearound an intermediate string of casing. The compressible particles aresuspended more densely central to the trapped annulus.

FIG. 5C is still another perspective view of the upper portion of awellbore. In this instance, a column of carrier fluid holdingcompressible particles is again placed in the unfilled annular spacearound an intermediate string of casing. The compressible particles aresuspended more densely along an upper end of the trapped annulus.

FIG. 6 is a flow chart showing steps for preparing a mixture ofcompressible particles for a wellbore annular area.

FIG. 7A is a top view of an illustrative compressible particle as may beused in an annular region of a wellbore, in a first embodiment.

FIG. 7B is a top view of an illustrative compressible particle as may beused in an annular region of a wellbore, in a second less porousembodiment.

FIG. 7C is a top view of an illustrative coating that may be used arounda compressible particle.

FIG. 8A is a top view of an illustrative compressible particle. Theparticle presents a geometry having a smooth outer surface and an ovalprofile.

FIG. 8B is another illustrative compressible particle. Here, theparticle has an irregular shape.

FIG. 9A is a sectional view of a carrier fluid wherein compressiblefluids are suspended. The carrier fluid has received a plurality ofregularly-shaped, polygonal compressible particles.

FIG. 9B is another sectional view of a carrier fluid whereincompressible fluids are suspended. In this view, a coating has beenapplied to the compressible particles. The coating is charged, causingthe particles to repel each other, thereby enhancing disbursement.

FIG. 10A is a cartesian chart showing compressibility of particles as afunction of pressure. This demonstrates a “compressibility response.”

FIG. 10B is a graph showing a pressure profile within the annular regionof a wellbore. Pressure is shown as a function of depth, both before andafter pressure build-up due to production operations.

FIG. 11A is an illustrative view of an annular region, representing asmall portion of an annular column for a wellbore. The annular regionholds a plurality of compressible particles suspended in an aqueouscarrier fluid. The compressible particles are designed to have a densitythat is lower than the density of the fluid, thereby enabling buoyancy.

FIG. 11B is an enlarged view of one of the particles from FIG. 11A. Itcan be seen that the collapsible particle is designed to be considerablylarger in size compared to weighting agent particles in the carrierfluid.

FIG. 12 is a flow chart that shows steps for tuning a buoyantcompressible particle, in one embodiment.

DETAILED DESCRIPTION OF CERTAIN EMBODIMENTS Definitions

For purposes of the present application, it will be understood that theterm “hydrocarbon” refers to an organic compound that includesprimarily, if not exclusively, the elements hydrogen and carbon.Hydrocarbons may also include other elements, such as, but not limitedto, halogens, metallic elements, nitrogen, oxygen, and/or sulfur.

As used herein, the term “hydrocarbon fluids” refers to a hydrocarbon ormixtures of hydrocarbons that are gases or liquids. For example,hydrocarbon fluids may include a hydrocarbon or mixtures of hydrocarbonsthat are gases or liquids at formation conditions or at surfaceconditions. Hydrocarbon fluids may include, for example, oil, naturalgas, coalbed methane, shale oil, pyrolysis oil, pyrolysis gas, apyrolysis product of coal, and other hydrocarbons that are in a gaseousor liquid state, or combination thereof.

As used herein, the terms “produced fluids,” “reservoir fluids” and“production fluids” refer to liquids and/or gases removed from asubsurface formation, including, for example, an organic-rich rockformation. Produced fluids may include both hydrocarbon fluids andnon-hydrocarbon fluids. Production fluids may include, but are notlimited to, oil, natural gas, pyrolyzed shale oil, synthesis gas, apyrolysis product of coal, oxygen, carbon dioxide, hydrogen sulfide andwater.

As used herein, the term “fluid” refers to gases, liquids, andcombinations of gases and liquids, as well as to combinations of gasesand solids, combinations of liquids and solids, and combinations ofgases, liquids, and solids.

As used herein, the term “wellbore fluids” means water, hydrocarbonfluids, formation fluids, or any other fluids that may be within awellbore during a production operation. Wellbore fluids may include aweighting agent that is residual from drilling mud.

As used herein, the term “gas” refers to a fluid that is in its vaporphase. A gas may be referred to herein as a “compressible fluid.” Incontrast, a fluid that is in its liquid phase is an “incompressiblefluid.”

As used herein, the term “subsurface” refers to geologic strataoccurring below the earth's surface.

As used herein, the term “formation” refers to any definable subsurfaceregion regardless of size. The formation may contain one or morehydrocarbon-containing layers, one or more non-hydrocarbon containinglayers, an overburden, and/or an underburden of any geologic formation.A formation can refer to a single set of related geologic strata of aspecific rock type, or to a set of geologic strata of different rocktypes that contribute to or are encountered in, for example, withoutlimitation, (i) the creation, generation and/or entrapment ofhydrocarbons or minerals, and (ii) the execution of processes used toextract hydrocarbons or minerals from the subsurface.

As used herein, the term “wellbore” refers to a hole in the subsurfacemade by drilling or insertion of a conduit into the subsurface. Awellbore may have a substantially circular cross section. The term“well,” when referring to an opening in the formation, may be usedinterchangeably with the term “wellbore.”

As used herein and unless specified otherwise, the terms “annularregion” “annulus,” “containment area,” or the like refer to the volumebetween an inner tubular member and an outer tubular member or wellborewall, the term “outer” meaning having a larger radius with respect tothe inner member. As used herein, the annular region is normallyisolated, trapped, or otherwise confined annular region, in that it isnot in or no longer in circulating fluid communication with an innerbore of the inner tubular member during an operational period for thewellbore. These terms as used herein typically refers to the annulusesbetween casing and/or liner tubular strings, or between a productiontubing string and a production casing or liner, above a seal bore orproduction packer, as when a well is constructed wellbore fluids withinthese types of annuluses typically become trapped and are no longercapable of circulating fluid therein or therefrom.

Description of Selected Specific Embodiments

FIG. 2 is a perspective view of a tubular body 200 of the presentinvention, in one embodiment. In a preferred aspect, the tubular body200 is deployed in a wellbore as part of a string of casing. Statedanother way, the tubular body 200 may be threadedly placed in serieswith a string of casing (such as casing string 140 of FIG. 1B).

The tubular body 200 is specifically designed to reside along an openannular region such as region 142. The tubular body 200 may be of astandard length for a pipe joint, such as 30 feet, 32 feet or even 40feet.

The tubular body 200 comprises an upper end 210 and a lower end 214. Inthe vernacular of the industry, the upper end 210 is the box end whilethe lower end 214 is the pin end. The box end 210 comprises internalthreads 212 that are configured to threadedly connect with the pin endof an immediately upper joint of pipe (not shown). Reciprocally, the pinend 214 is configured to “stab” into the box end of an immediately lowerjoint of pipe (not shown) for threaded connection.

The tubular body 200 defines an elongated wall forming a pipe 220 (orelongated pipe body). Placed along the outer diameter of the pipe 220 isa filter screen 230. The filter screen 230 has an upper end 232 and alower end 523 and is designed to contain a plurality of compressibleparticles. An example of particles within the screen 230 is shown at 240in FIG. 2B, discussed further below.

In one aspect, the filter screen 230 has both an inner surface 235 andan outer surface 236, forming a defined cylindrical body. The innersurface 235 resides closely along the pipe body 220. The filter screen230 may be welded onto the pipe 220. Alternatively, the filter screen230 may be secured to the pipe 220 through a friction fit or by anadhesive to form a cylindrical body. In another embodiment, the screen230 does not have an inner surface, but is securely fastened to theouter diameter of the pipe body 220 itself to contain the compressibleparticles 240.

The filter screen 230 is fabricated from a porous material. In theembodiment shown in FIG. 2A, the material is a permeable polymericmaterial having micro-pores or slots that form a mesh. Suitablepolymeric materials may include neoprene, polyurethane rubber, vinyl,nitrile rubber, butyl rubber, silicone rubber, or combinations thereof.Alternatively, and as described further below in connection with FIGS.3A and 3B, the material may be a metal alloy or a ceramic materialhaving pre-fabricated micro-slots.

FIG. 2B is a perspective, cut-away view of the screen 230 of FIG. 2A.The screen 230 is again fitted onto an elongated pipe body 220. The pipe220 may be deployed in a wellbore as part of a string of casing. Theprocess of fitting the annular screen 230 onto the pipe joint 200 mayinvolve wrapping the screen 230 around the pipe body 220 and thensecuring it through friction or heat melding. More preferably, theprocess involves sliding the annular screen 230 onto the pipe body 220(over the pin end 214) and then welding the screen 230 onto the pipebody 220. In this instance, the screen 230 is fabricated from a metalmaterial or at least has metal frame members or ribs for welding.

In FIG. 2B, an inner bore 205 of the pipe 220 can be seen. The pipe 220may be fabricated from any steel material having burst and collapsepressure ratings suitable for a wellbore environment. Those of ordinaryskill in the art will understand that with the advent of hydraulicfracturing, burst ratings of pipe (and particularly of productioncasing) are much higher than in older wells and may withstand pressuresof up to 15,000 psi. Of course, when welding or otherwise securing thescreen 230 onto the outer diameter of the pipe body 220 care must betaken not to compromise the integrity of the joint 200 as a pressurevessel by scoring the pipe 220.

As an alternative, the pipe body 220 may be fabricated from ceramic. Inthis instance, the screen 230 is preferably secured to the pipe body 220through a mechanical connection such as a latch or raised surfaces.

An encased area 238 is provided within the screen 230. As noted above,the encased area 238 holds a plurality of compressible particles 240.Preferably, a sufficient number of compressed particles are used to fill(or substantially fill) the encased area 238. The outer surface 236 ofthe filter screen 230 and the compressible particles 238 are designed tobe sturdy enough to hold a cylindrical shape around the pipe body 220until a designated annular pressure is reached. Once the designatedannular pressure is reached, the compressible particles 240 will beginto collapse, thereby absorbing pressure within the annular pressure andreducing the likelihood of the pipe 220 collapsing during productionoperations.

As noted, the screen 230 may be fabricated from a polymeric materialhaving micro-pores. In this instance, when the designated annularpressure that causes compression or collapse of the compressibleparticles 240 is reached, the porous, polymeric body 236 may at leastpartially collapse around the compressed particles 240. Preferably, thecollapsibility response (or pressure rating) of the particles 240 isless than the collapsibility response (or pressure rating) of the screen230, though this is a matter of engineer's choice.

FIGS. 3A and 3B present alternate embodiments of an annular filterscreen 230. FIG. 3A presents the screen as a wound filter screen 300A.The filter screen 300A is similar to a known sand screen. The filterscreen 300A may be fabricated from either steel (or anycorrosion-resistant alloy) or ceramic. Preferably, the filter screen300A is fabricated from metal wire 310A that is wound around andsupported by elongated vertical ribs (not visible). Micro-slots 315A arepreserved between the wire 310A to enable pressure communication intothe containment area 238.

FIG. 3B presents the filter screen 300B as a slotted tubular. The filterscreen 300B defines a metal tubular body 310B with a plurality ofdedicated slots 315B. The slots 315B again enable pressure communicationinto the containment area 238.

Each filter screen has an upper end 312 and a lower end 314. The filterscreens 300A or 300B are designed to be fitted around an outer diameterof the pipe 220 and filled with compressible particles 240. Each filterscreen 300A or 300B will present slots 315A, 315B that permit fluid andpressure communication between the wellbore and the compressibleparticles. The gap size of the slots 315A, 315B in the screens 300A,300B may range in size from 10 μm to 100 μm, depending on the specificparticle size distribution. At the same time, the particle sizedistribution will be between 10 μm and 700 μm (dry). It is understoodthat the gaps 315A, 315B must be smaller than the diameters of thecompressible particles 240.

Use of the screen 230 (or screens 300A or 300B) enables the delivery ofthe compressible particles 240 within a “trapped annulus.” In thisrespect, it is not necessary to pump compressible particles 240 ahead ofthe cement column (e.g., column 145) for placement within the annulararea (e.g., annular area 142). Extending the length of the screen 230and/or increasing the density of the particles 240 within thecontainment area 238 and/or using multiple tubular bodies 200 increasesthe pressure absorption abilities within a trapped annulus 142, 152.

It is also noted that the use of the tubular body 200 with a screen 230,300A or 300B enables the operator to place the particles 240 in aspecific location in the trapped annulus. For example, the operator maydesire to keep the compressible particles central to the trappedannulus. In this instance, the operator may place one or more tubularbodies 200, in series, generally half way between the top and the bottomof the fluid column making up the trapped annulus.

It is preferred that the filter screen 230, 300A or 300B cover about 80%of the length of the pipe body 220. The operator may place one, two, oreven ten tubular bodies 200 having the filter screen 230, 300A or 300Balong an annular region 142. The tubular bodies 200 may be connected inseries, or may be spaced apart by placing standard casing joints betweentubular bodies 200.

FIG. 4 is a perspective view of the upper portion of a wellbore 400. Thewellbore 400 is in accordance with the wellbore 100 of FIG. 1B. In thisrespect, the wellbore 400 is completed with a series of casing stringsincluding surface casing 120, intermediate casing strings 130 and 140,and production casing 150.

In FIG. 4, Arrow F is shown. This indicates a flow of production fluidsduring a hydrocarbon production operation. It is understood that theproduction fluids are produced through a production string (not shown).The production fluids “F” are warm, causing a temperature within thevarious annular regions 132, 142, 152 to increase. This, in turn, willincrease the temperature of the fluids within these annular regions 132,142, 152. The increase in temperature within the defined volumes willcause a corresponding increase in pressure.

To accommodate the pressure increase within the trapped annulus 142, aseries of novel tubular bodies 200 is provided. Each body includes apermeable filter screen 230 containing a plurality of compressibleparticles 240. The particles 240 are preferably fabricated fromcompressive carbon beads such as mesocarbon micro-beads. Mesocarbonmicro-beads (“MSMB's”) represent a porous graphite carbon (PGC) materialwherein an inner core is composed of amorphous carbon, while an outershell is composed of graphitic carbon. Both the inner core material andthe outer shell material are porous.

MCMB's may be fabricated from coal tars. In one aspect, a surface-coatedmicro-bead material may be produced by carbonizing thermosetting resin.Such beads are available under the trade name NicaBeads® and areproduced by Nippon Carbon Co. Ltd. of Tokyo, Japan.

Other materials may also be used for the compressible particles 240. Forexample, a composite of polymer and graphite may be formed into beads.The graphite material may include graphite carbons. Such materials areavailable from Superior Graphite Co. of Chicago Ill. Alternatively,graphene beads having a high porosity to enhance compressibility may beused. Pore channels within the beads may optionally be coated withnatural rubber or a polymer or pseudo-polymer serving as a syntheticrubber.

In one arrangement, flexible compressible beads comprised of a polymericmaterial are used. For example, a co-polymer of methylmethacrylate andacrylonitrile may be used. Styrofoam or polystyrene may also be usedalone or in combination with this co-polymer. In another embodiment, aterpolymer of methylmethacrylate, acrylonitrile and dichloroethane isused. The dichloroethane may be a vinylidene dichloride. Preferably, thebeads are not infused with gas so as to limit expansion of the beadmaterial upon exposure to heat during wellbore operations.

Other polymeric materials may be used such as neoprene, polyurethanerubber, vinyl, nitrile rubber, butyl rubber, EPDM rubber, siliconerubber, or combinations thereof. The material may be continuous or itmay be porous, having a porosity of 5% to 40%, or more preferablybetween 10% and 20%. It is understood that the above materials aremerely illustrative.

Preferably, the particles will have a compressibility of between 10% and30%. More preferably, the particles will have a compressibility ofbetween 14% and 22% (up to 10,000 psi).

Preferably, each of the particles has a resiliency of between 80% and120%. More preferably, each of the particles has a resiliency of between87% and 117%.

As noted, the particles 240 are confined within a containment area(shown at 238 in FIG. 2) defined by a filter screen. In FIG. 4, thewellbore 400 has received a series of tubular bodies 200, each having anelongated filter screen 230. While the screen is indicated as element230, the screen may alternatively be filter screen 300A of FIG. 3A or300B of FIG. 3B. The use of a filter screen 230 allows the operator toselect the depth at which the particles 240 are placed along the trappedannulus without having to worry that the particles may float to the topof the column or settle to the bottom of the column along the trappedannulus 142. Stated another way, the operator can use particles 240having a desired compressibility without worrying about bed heights atthe bottom or the top of the annulus 142. Since the particles 240 arecontained, the bed height is generally pre-determined by the height ofthe filter screen 230 and the number of tubular bodies 200 employed inseries. Further, the operator may be less concerned with particledensity since buoyancy is not a factor.

In the embodiment described in FIG. 4, the compressible particles 240are contained within one or more annular screens 230 placed around apipe joint 220, (e.g., a joint of casing). However, an alternativesolution to alleviating pressure build-up in an annular area (such asarea 142) is to use collapsible particles dispersed in the fluidresiding along the fluid column in the annular area. Once again, theparticles are volumetrically compressed as the pressure increases duringproduction operations, resulting in additional volume into which thefluid can expand. This may be in lieu of or in addition to use oftubular bodies 200.

FIG. 5A presents a cross-sectional view of the upper portion of awellbore 500A. The wellbore 500A extends into a subsurface formation510. As with wellbore 400 of FIG. 4, wellbore 500A has been completedwith a series of casing strings. These include a surface casing string520 and at least two intermediate casing strings 530 and 540. Inaddition, a production casing 550 is used, providing for a “cased hole.”

Annular area 525 resides around the surface casing string 520.Similarly, annular areas 535 and 545 reside around casing strings 530and 540, respectively. Additionally, annular area 555 resides aroundproduction casing 550. Cement is placed around the lower portions ofannular areas 535 and 545. Preferably, cement is also placed around atleast a lower portion (not shown) of annular area 555 up to a bottom ofcasing string 540. In the illustrative view of FIG. 5A, only annulararea 525 is completely filled with cement.

It is observed that in the wellbore 500 no annular screen is used as inFIG. 4; instead, a column of carrier fluid 543 holding compressibleparticles 546 is used. The carrier fluid 543 resides in an annular area542 around the production casing 540, forming a fluid column or bed. Thecarrier fluid 543 is placed in what will otherwise be a “trappedannulus” 542 above a column of cement 545.

In the arrangement of FIG. 5A, the compressible particles 546 aresuspended in and dispersed somewhat evenly along the column of carrierfluid 543. During production, hydrocarbon fluids are lifted to a surface505 in accordance with Arrow F through a production tubing (not shown).Formation fluids may flow to the surface 505 under in situ pressure;alternatively, formation fluids may be raised to the surface 505 usingan artificial lift technique. In either instance, as formation fluidsare produced according to Arrow F, the temperature in the wellbore 500Awill increase. This, in turn, will increase pressure within the trappedannulus 542 due to fluid expansion.

To alleviate this pressure and to protect the adjacent casing strings530, 540, the particles 546 are volumetrically compressed. This resultsin additional volume into which the fluid can expand as the pressureincreases during production operations.

To maximize the effectiveness of the compressible particles 546, it isideal if all of the particles 546 are exposed to pressure within thetrapped annulus 542 equally. In this way the particles 546 can compressproportionally. This may not be achieved if all of the particles 546rise together to the top of the trapped annulus 542. In this respect, atleast some of the particles 546 will be lost during the periodic annulusbleed downs that occur during production operations.

Similarly, it may be undesirable for the particles 546 to settletogether at the bottom of the fluid column 543, forming a bed. Such abed would represent a collection of particles 546 which, depending onthe number of particles used and the height and area of the annulus 542,could prevent pressure contact across all of the fluid column 543.Stated another way, fluid pressure may not fully penetrate through theentirety of the bed height. In addition, if the particles 546 settletightly anywhere along the column 543 they could build an impermeablebridge resulting in trapping that, without the addition of thecompressible particles 546, was an open annulus along the column 543,thus creating a problem where previously none had existed.

To ensure that the particles 546 remain well-dispersed along the annularcolumn 543, consideration should be given to the density of theparticles 546 relative to the carrier fluid 543. A lower density can beachieved by designing large particle size, increased pore volume, orproviding random shapes that mitigate packing. It is believed thatirregular particle shapes with higher surface area allows for bettersuspension in the fluid column. Preferably, the density of the carrierfluid 543 is between 12 ppg and 12.8 ppg (1.43 g/cc to 1.54 g/cc) andthe densities of the compressible particles 546 span across this range.In one aspect, the particles 546 may range in density from 0.5 to 2.5specific gravity. A uniform suspension of particles 546 can be achievedby designing the carrier fluid 543 density to generally match thedensity of the particles 546 (or vice versa).

The particles 546 are preferably fabricated from a compressive carbonsuch as mesocarbon micro-beads or graphite as described above.Alternatively, a composite of polymer and graphite may be formed intobeads.

In one arrangement, flexible compressible beads comprised of a polymericmaterial are used. For example, a co-polymer of methylmethacrylate andacrylonitrile may be used. Styrofoam or polystyrene may also be usedalone or in combination with this co-polymer. In another embodiment, aterpolymer of methylmethacrylate, acrylonitrile and dichloroethane isused. The dichloroethane may be a vinylidene dichloride. Preferably, thebeads are not infused with gas so as to limit expansion of the beadmaterial upon exposure to heat during production operations.

Consideration should also be given to particle size. The compressibleparticles 240 may range in size from 10 μm to 700 μm in diameter, andmore preferably between 40 μm and 700 μm. In one aspect, at least 50% ofthe compressible particles have a diameter range that is from 50 μm to600 μm, with an average size that is between 200 μm and 400 μm (in drystate).

The slots 310A or 310B are sized to contain the particles 546 whileallowing ingress of fluid and pressure. During operation, pressure needonly migrate from the O.D. of the screen 300A or 300B where the fluid ispresent to the O.D. of the pipe joint 220.

FIG. 6 is a flow chart showing steps for a method 600 of designing amixture having compressible particles. The mixture will be comprised ofthe carrier medium 543 and the compressible particles 546. The aim ofthe method 600 is to provide for a desired distribution of particles 546across a trapped annulus 542.

The method 600 first includes selecting a carrier fluid. This is shownin Box 610. The carrier fluid is preferably an aqueous liquid comprisedprimarily of fresh water, salt water or brine. Water based drillingfluid may also be considered wherein the drilling fluid comprises asmall sampling of weighting agent. Weighting agent particles are shownat 1130 in FIG. 11B.

Fresh water, of course, has a specific gravity of 1.0. Where salt orminerals are present, the specific gravity will be increased. Thecarrier fluid may need to be blended to ensure a generally homogenouscomposition and specific gravity.

The method 600 next includes selecting a density range for thecompressible particles 546. This is provided in Box 620. The particles546 may have a range in density from 12.0 ppg to 12.8 ppg. Ideally, thecompressible particles 546 will have a specific gravity (“SG”) that isclose to that of the carrier fluid 543. Preferably, the SG of thecompressible particles 546 will have a range of plus/minus 0.5 of thecarrier fluid 543. This will prevent particles from settling at thebottom or rising to the top of the narrow annulus 542, forming a bedthat isolates the annulus (or at least many of the particles) frompressure.

In the narrow confines of an annulus 542, forming a dense particle bedat the top or bottom of the annulus could be detrimental. In thisrespect, as the particles bridge off they will limit the fluid'spressure penetration from one end of the bed to the other end of thebed. The consequence is that not all particles would be compressed asthe pressure increases in the trapped annulus 542. If all of theparticles 546 settle at the bottom of the column 543 where an open shoewas present, the bed could restrict pressure distribution up into thefluid column 543.

Notwithstanding the arrangement of FIG. 5A, in some instances theoperator may want to cluster a majority of the particles 546 central tothe trapped annulus 542. FIG. 5B is another perspective view of theupper portion of a wellbore 500B. The wellbore 500B is constructed inaccordance with the wellbore 500A of FIG. 5A and extends into thesubsurface formation 510. A column of carrier fluid 543 holdingcompressible particles 546 is again placed in the unfilled annular space542 around an intermediate string of casing 540. The compressibleparticles 546 are suspended together within and dispersed along thecolumn of carrier fluid 543.

In FIG. 5B, the concentration of particles 546 is higher along a centralportion 546B of the annular region 542 than it is near the bottom. Thisis due to an intentional variation of the density of the particles 546as mixed in the carrier fluid 543. One way of adjusting the location ofthe particles 546 along the trapped annulus 542 is by increasing ordecreasing the porosity of the particles. Some particles having a lowerdensity are included, which will generally rise to the top of the fluidcolumn 543, while some particles having a higher density will beincluded which will generally settle to the bottom of the fluid column543. In the arrangement of FIG. 5B, the particles 546 are generallydispersed along the fluid column 543, but a higher percentage ofparticles have a SG that approximates the density of the carrier medium543, causing most of the particles 546 to settle along the centralportion 546B.

FIG. 7A is a top view of an illustrative compressible particle 700A asmay be used in an annular region 542 of a wellbore, in a firstembodiment. Here it can be seen that the particle 700A has a number ofholes 710. The holes 710 increase porosity and create buoyancy. Theholes 710 may further enhance compressibility response.

The compressible particle 700A shown in FIG. 7A is preferably a carbonor carbon-based material. More preferably, the material comprisesgraphene, representing carbon material placed in layers. Grapheneparticles are shown to provide low-density, high compressibility andhigh elasticity. In addition, graphene particles can have resistance tofatigue. In one aspect, a carbon-graphene composite compound of archedstructures arranged into parallel stacks is used.

FIG. 7B is a top view of an illustrative compressible particle 700B inan alternate embodiment. In this arrangement, the particle 700B hasfewer holes 710, indicating a lower porosity (or higher density). Again,the particle 700B may be a carbon-graphene composite.

With reference again to FIG. 5B, particles in accordance with particle700A and particles in accordance with particle 700B are used in thecarrier fluid 543. Particles 700A have a lower relative density and areclustered near the top of the annular column while particles 700B have arelatively higher density and settle more readily along the fluid column543, extending to the bottom.

It is noted that the illustrative particles 700A and 700B have irregularprofiles. The irregular profiles are polygonal, or multi-sided.Particles having a variety of outer diameters, profiles and specificgravities may be employed to prevent excessive packing duringpressurization. This enables pressure communication with the entireouter surface of each particle 700A, 700B.

FIG. 7C is a side view of a coating 700C that may optionally be usedaround a compressible particle, such as particle 700A or 700B. Thecoating 700C may be applied by dipping the particles 700A, 700B into avat containing a liquefied coating material. The coating material maycomprise, for example, natural rubber, synthetic rubber or athermoplastic elastomer. An example of a thermoplastic elastomer isvinylidene fluoride-hexafluropropylene co-polymer. The coating will haveits own porosity, aiding in the compressibility and buoyancy of theparticles 700A, 700B.

As an alternative to the coating process, compressible particles 700A,700B may be placed on a screen or tray and passed under a spray ofwarmed elastomeric coating material. As the particles 700A, 700B passthrough or under the spray of the warmed elastomeric coating material,the coating 700C is applied to the compressible particles 700A, 700B.The particles 700A, 700B are then gently shaken or mixed to ensure thatthe particles 700A, 700B do not adhere to each other as the polymercoating dries. The particles 700A, 700B may be passed through a coolingstation to congeal the polymeric material, or may be run through adrying station wherein blowers are used to blow air across thecompressible particles 700A, 700B.

Returning again to FIG. 6, the method 600 additionally includesselecting a geometry for the compressible particles. This is seen in Box630. Geometry refers to both shape and size.

The compressible particles may range in size from 10 μm to 700 μm indiameter, depending on the specific particle size distribution. Morepreferably, the particle size distribution will be between 100 μm to 700μm or even between 200 μm and 400 μm.

Particles having a variety of outer diameters and a variety of specificgravities are preferably employed to prevent packing duringpressurization. In one aspect, particles have irregular sides such asshown with particle 700A are used. In another aspect, particles havingdifferent shapes may be employed to prevent packing. For example, someparticles may have a circular profile while others may have an ovalprofile.

FIG. 8A is a top view of a compressible particle 800A. The particle 800Apresents a geometry having a smooth outer surface. In this view, theprofile is oval. As an alternative, the profile could be circular.

FIG. 8B is a top view of another illustrative compressible particle800B. Here, the particle 800B has an irregular shape. Particle shape maybe adjusted to selectively increase or decrease the degree to whichparticles “fit together” while undergoing compressive forces downhole.

FIG. 9A is a sectional view of a carrier fluid 920. In this view, acollection 900A of compressible particles 910 is suspended in thecarrier fluid 920. Of interest, the illustrative particles 910 share thesame shape, each having a polygonal profile. The particles 910 arecompressed together due to pressure build-up within a trapped annulus.

Here, the particles 910 are compressed into an agglomeration. This is inresponse to compressive forces acting within a trapped annulus.

FIG. 9B is a second illustrative view of a collection 900B of thecompressible particles 910 in a carrier medium 920. Here, the particles910 each have a coating 930. Either the particles themselves 910 or thecoating 930 includes an electrical or magnetic charge that causes theparticles 910 to repel one another. This inhibits packing of theparticles 910 within the wellbore. This also inhibits settling of theparticles 910 within the annulus, enabling the particles 910 to remainwell dispersed along the fluid column 543 in spite of pressure build-up.Stated another way, the coating 930 mitigates formation of a pressureimpenetrable bed after particle settlement.

The coating 930 may be applied during finishing operations of thecollapsible particle manufacturing as to not disrupt the mechanicalproperties and structure of the particles 910. The coating 930 may beengineered to ensure compatibility such that interaction of surfacecoating chemicals and downhole chemicals used in the carrier fluid 920is not detrimental and so that the coating 930 maintains its utilityduring lifecycle of particle utilization.

Engineering the collapsible particle size and shape will allow forbuoyancy in the fluid column. Smaller particle size and irregularparticle shape with higher surface area will allow for better suspensionin the fluid column 543.

Returning to FIG. 6, the method 600 further includes selecting acompressibility response for the compressible particles. This is shownin Box 640. Compressibility may be measured in terms of volumetricchange per pressure change (dV/dP). Preferably, the compressibilityresponse is between 0.5 mm and 1.0 mm/100 psi. Alternatively, each ofthe compressible particles has a compressibility of between 14% and 22%(up to 10,000 psi). The particles may also have a resiliency of between87% and 117%.

FIG. 10A is a Cartesian chart 1000A showing compressibility of aparticle. Compressibility is indicated along the y-axis as a percentageof volumetric change, while pressure (measured in psi) is shown on thex-axis. Line 1000 demonstrates a compressibility response of a particleas pressure increases. To maximize the effectiveness of compressibleparticles, the pressure acting on those particles ideally would bewithin the area of a compressibility curve that maximizes the volumetricchange per pressure change (dV/dP). In FIG. 10A, this resides withinP_(A) and P_(B).

The compressible particles, when suspended in a carrier fluid, should beplaced within the annulus such that the predicted pressure P at theposition of placement is within the maximum dV/dP capabilities of theparticle. This would be within the range between P_(a) and P_(b). P_(a)would be the initial pressure state of the annulus before the annulusbuilds up pressure. P_(b) represents a final pressure state of theannulus after productions operations have commenced and the wellbore haswarmed.

The depth of this pressure range P_(a)-P_(b) can be found by calculatingthe expected pressure profile within the annulus. The end result of thisis that compressible particles 546 are placed to maximize theeffectiveness of their compressibility response.

FIG. 10B is a graph 1000B showing a pressure profile within the annularregion of a wellbore. Vertical depth within the annulus is shown on they-axis, measured in feet, while pressure in the annulus is shown on thex-axis, measured in psi. Once again, the pressure values P_(A) and P_(B)are indicated, meaning pressure both before and after pressure build-up.

Two different depths are shown in FIG. 10B, referenced as Depth 1 andDepth 2. Depth 1 indicates an upper portion of a trapped annulus whileDepth 2 indicates a lower portion of a trapped annulus. Depth 2 isobviously lower than Depth 1.

As depth within the annulus increases, the pressure increases. Thissuggests that the depth of particle placement should be tuned tomaximize the compressibility response of the particles. Thus, particleswith a higher degree of compressibility should be placed closer to Depth1 while particles with a lower degree of compressibility should beplaced closer to Depth 2.

As an aside, where screens (such as screen 230) are used, the operatormay choose to place particles having a higher degree of compressibilityin screens residing at shallower locations along the wellbore.Reciprocally, the operator may choose to place particles having a lowerdegree of compressibility in screens residing along joints that are ator deeper locations along the wellbore.

Where the filter screens are not used, the operator may use staged fluiddisplacements to place particles having different compressibilitiesalong the annular region. This means that the operator will pump downcarrier fluid carrying particles having a higher degree ofcompressibility first, followed by particles having a slightly lowerdegree of compressibility second, followed still by particles having aneven lower degree of compressibility third, and so forth. Preferably, nomore than three stages would be employed.

As another option, the particles may have different densities,corresponding to their compressibility responses. Some particles willhave a lower density and a higher compressibility response. Still otherswill have a slightly lower density and a slightly higher compressibilityresponse. At the same time, some particles will have the highest densityand the lowest compressibility response. All of these particles may bemixed together (shown at Box 650) before pumping.

Once the carrier fluid 543 is placed within the annulus 542, somere-settling of particles 546 will take place. Particles 546 having thelowest density will slowly rise to the top of the column 543 whileparticles 546 having the highest density will slowly settle towards thebottom.

In one aspect, particles having higher degrees of compressibility willbe designed with a lower density. Similarly, particles having lowerdegrees of compressibility will be designed to have a higher density. Inthis way, all compressible particles may be pumped into the annularregion together ahead of a cement slurry, with the understanding thatthe particles will at least partially re-settle themselves according totheir respective densities. Density may be adjusted, for example, byincreasing or decreasing porosity.

As a third option, the operator may choose to deliberately placeparticles within an annular region 542 in stages. Particles 546 with alower density and a higher compressibility response will be pumped downfirst. This would be followed by particles 546 with a slightly lowerdensity and a slightly higher compressibility response. Particles 546having the highest density and the lowest compressibility response wouldbe pumped down last, just ahead of the cement. In this arrangement, twoto five stages of fluid displacement may be employed.

In any instance, where a carrier medium 543 carrying compressibleparticles 546 is pumped down a wellbore (such as wellbore 500B), theoperator will need to mix the particles into the fluid first. This isprovided at Box 650. Preferably, the compressible particles 546 aremixed into the fluid 543 at a concentration of 5% to 40% by volume. Thegreater the concentration of particles there is, the greater the overallcompressibility the fluid column 920 will have. Thus, overallcompressibility is impacted not only by the degree of compressibility ofthe particles along the column 543, but also by the number of particles546 provided.

In connection with the mixing step of Box 650, the operator may chooseto add additives to the mixture in order to increase the rheologicalproperties (e.g., plastic viscosity, yield point value, and gelstrength) of the mixture. Such additives may include one or more naturaland/or synthetic polymeric additives, polymeric thinners or flocculants.The purpose of such additives is to alter the gel strength of the fluid543 to inhibit particle settling.

Alternatively, the operator may provide an electric or magnetic chargeto the particles to keep them suspended. Alternatively still, theparticles may be coated with a material having an electrical or magneticcharge to inhibit settling. Such an arrangement is shown in FIG. 9Bdiscussed above.

It is observed here that the presence of gas in the carrier fluid 543can inhibit the performance of the compressible particles 546. Toimprove the performance of the compressible particles 546, the carrierfluid 543 may be prepared such that the gas content is minimized. Thus,a step 660 of de-gasifying the mixture may be provided for the method600.

One option for reducing gas content is through mechanical agitation orstirring. For example, the motor and impeller of a known “gas trap”apparatus may be employed. An example of such a gas trap is theFloatair® gas trap available from Floatair LLC of Carlsbad, N. Mex.Details concerning this gas trap are presented in U.S. Pat. No.9,879,489, and are incorporated herein by reference. The gas trap may befloatably placed within a mixing tank, causing vapors to escape throughthe top of a mixing canister.

As another option, the mixture may be carried from a mixing tank that isheld under pressure to a holding tank having less pressure, such as anopen air tank. To aid in degasification, sonic energy may be applied toagitate the mixture through pulses that drive the gas out. For example,a mega-sonic energy transducer that generates between 900 kHz and 2.0MHz may be used.

Alternatively or in addition, a hold time could be applied to themixture before pumping into the wellbore in order to allow gases to comeout of solution at ambient conditions. Additionally, gas content in thecarrier fluid 543 may be reduced by applying a reductant, or reducingagent.

Referring back to FIG. 6, the method 600 may also optionally includeadjusting particle density to compensate for the presence of a weightingagent. This is indicated at Box 670.

Those of ordinary skill in the art will understand that during thedrilling of a wellbore, a weighting agent is typically used as part ofthe drilling fluid. The weighting agent increases the density of thefluid and, thereby, increases the hydrostatic head acting down on thedrill bit and the surrounding formation as the drill bit rotates andpenetrates downhole. The weighting agent helps act against highformation pressures that may “kick” into the wellbore. The weightingagent also forms a “cake” against the wellbore wall to prevent fluidloss during circulation.

Commonly-used weighting agents include barite and hematite. Weightingagent particles will remain in the wellbore after casing strings are runinto the hole. This means that as the fluid mixture (that is, thecarrier fluid 543 with compressible particles 546) is pumped downhole,the mixture will likely pick up weighting agent particles en route tothe annular area 542. This will affect the buoyancy of the compressibleparticles 546.

FIG. 11A is an illustrative view of an annular region 1100. The annularregion 1100 represents a small portion of an annular column for awellbore, such as the fluid column 543 of FIG. 5A. The annular region1100 holds a plurality of compressible particles 1110. The particles1110 are suspended in an aqueous carrier fluid 1120. The compressible(or collapsible) particles 1110 are designed to have a density that islower than (or very close to) the density of the fluid 1120, therebyenabling buoyancy.

In the column 1100 of FIG. 11A, weighting agent particles 1130 can alsobe seen. Due to their weight, most of the particles 1130 have settledtowards the bottom of the column 1100. At the same time, some of theweighting agent particles 1130 have attached themselves to thecompressible particles 1110, causing the compressible particles 1110 toalso sink towards the bottom of the column 1100. The weighting agent1130 may be any weighting agent known to be used in drilling mud, suchas barite or hematite.

To avoid this scenario, the density of the particles 1110 can beengineered and adjusted. FIG. 11B is an enlarged view of one of theparticles 1110 from FIG. 8A. It can be seen that the collapsibleparticle 1110 is designed to be about 100 times larger in size comparedto the weighting agent particles 1130. The large difference in theparticle size between the compressible particles 1110 and the weightingagent particles 1130 reduces the amount of drag/sag during solidssettlement, and reduces the concentration of the compressible particles1110 in the annular fluid 1120.

In some instances, the presence of the weighting agent particles 1130 isbeneficial, particularly for compressible particles 1110 that have lowercompressibility. In this case it is desirable for the particles 1110 tosettle lower along the column 543. A weighting agent will help keep theparticles within the desired range of depth for maximum compressibilityas discussed above in connection with FIG. 10B, that is, between Depths1 and 2.

For particles with a higher compressibility, a lower density can beachieved by designing the larger particles 1110 to have increased porevolume. In this way, the particles 1110 stay buoyant in the fluid 1120even in the presence of weighting agent 1130 particles. The density ofthe collapsible particles 1110 should be lower than that of theweighting agent particles 1130. The collapsible particles 1110 may alsobe designed to have limited affinity to the weighting agent particles1130.

This issue of drag caused by the weighting agent particles 1130 can befurther mitigated by applying a coating (shown at 700C in FIG. 7C) tothe particles 1110. The coating 700C will serve as a wetting agent orenhanced particle disperser. Collapsible particleclogging/conglomeration is thereby reduced in low flow through andsuspension volume areas. The use of a coating 700C may also increaseuniformity in compressible particle distribution in suspensionoperations. Increased distribution of collapsible particles 1110 throughthe annulus 542 would not significantly increase kinetic energy,temperature build, or pressure build.

It is again observed that if the particles 1110 cannot be suspended inthe annular fluid, the particles will either settle down in the column1100 or settle (or float) up in the column 1100, depending on therelative specific gravities of the fluid 1120 versus the compressibleparticles 1110. Settling down results in a state where the weight of theparticles 1110 is pulling the bed into a more compacted state. Settlingup results in a situation where the weight of the particles 1110 ispushing the bed into a less compacted state. This results in a higherlikelihood of pressure penetration from one side of the particle bed tothe other side of the particle bed.

The operator may therefore prefer a configuration where a bed ofparticles 546 is used that intentionally floats to the top of theannulus 542. For this, the particle density can be engineered to ensureit is buoyant in the carrier fluid 543. For example, this can beachieved by modifications to particle shape, size, porosity, morphology,texture, and material.

FIG. 5C is another perspective view of the upper portion of a wellbore500C. In this instance, a column of carrier fluid 543 holdingcompressible particles 546 is again placed in the unfilled annular space542 (or trapped annulus) around an intermediate string of casing 540.The compressible particles 546 are dispersed along the fluid column 543.However, in this view the concentration of particles 546 is greateralong an upper portion 546C of the column of carrier fluid 543. This maybe because the fluid mixture pumped into the annular region 542 had ahigher concentration of compressible particles 546 at the beginning ofthe pump stage than at the end. Alternatively or in addition, this maybe because the particles 546 at the top of the column 543 are tuned tohave a higher degree of buoyancy.

Placing a majority of the compressible particles 546 at or near theupper end of a trapped annulus 542 allows the column of fluid 543 withinthe trapped annulus 542 to expand upward against the compressibleparticles 546. This results in a higher probability that the pressurewill migrate through the bed, maximizing the amount of particles thatare compressed.

Regardless of location within the column 543, the density of theindividual particles 546 should be lower than the density of the carrierfluid 543 to enable buoyancy. Particle density may be tuned byincreasing or decreasing porosity, increasing or decreasing particleshape or size, or by selecting a lighter or heavier carbon material. Ingeneral, buoyancy may be increased by increasing porosity (such as isshown in FIG. 7A) and by selecting a lighter carbon material.Compressibility may be enhanced by increasing porosity and by providingan elastomeric coating along either pore channel walls or the outerdiameter (such as is shown in FIG. 7C). Pressure penetration may beenhanced by providing a randomness to the particle profiles (such as isshown in FIGS. 7B and 8B) which prevents packing.

It is observed from FIG. 5C that as the compressible particles 546 risealong the column 543, any weighting agent particles (shown at 1130 inFIGS. 11A and 11B) will settle out below the compressible particles 546.In some instances, weighting agent particles 1130 will settle to thebottom as demonstrated in FIG. 11A. A fraction of the lighter,collapsible particles 546 (or 1110 as shown in FIG. 11A) may also bepulled down with the barite particles 1130 to form a compacted bed withthe barite particles 1130. This phenomenon may be referred to as barite“sag.”

Barite sag will have the effect of dragging at least a small portion ofthe compressible particles 546 to the bottom of the annular region 542.This both reduces the dispersion of particles 546 along the fluid column543 and creates an undesirable packing of particles at the bottom of thecolumn 543. To minimize this, and as noted above, at least some of thecollapsible particle 1110 may be designed to be about 100 times largerin size compared to the weighting agent particles 1130. The largedifference in the particle size between the compressible particles 1110and the weighting agent particles 1130 reduces the amount of drag/sagduring solids settlement. The difference in size, shape, particlemorphology and density between the barite and collapsible/compressibleparticles is such that fluid pressure can still reach up through thecolumn 543 and to the compressible particle 546 at the top even aftersolid settlement in the lower regime in the column 543.

With respect to each of FIGS. 5B and 5C, it is noted that each of areas532 and 552 is also a trapped annulus. However, these annuli 532, 552are not shown as having compressible particles 546. The operator may addcompressible particles in these annuli 532, 552 using the method 600described in FIG. 6. On this note, FIG. 6 finally includes the step ofpumping the mixture (that is, the carrier fluid with suspendedcompressible particles) into a wellbore annulus. This is seen in Box680. The carrier fluid is pumped in ahead of the cement column.

As can be seen, a unique collection of compressible particles isprovided. The compressible particles are intended to be used forattenuating pressure within a confined volume such as a trapped annuluswithin a wellbore. In addition, a unique method of designing (or“tuning”) compressible particles for a fluid mixture is also provided.

FIG. 12 is a flow chart 1200 that shows steps for tuning compressibleparticles, in one embodiment. As a first step, the method 1200 firstcomprises determining a density of a carrier fluid. This is shown in Box1210. The carrier fluid is preferably an aqueous fluid. The aqueouscarrier fluid may be fresh water, brine or an aqueous drilling fluid.

The method 1200 further includes selecting a density for thecompressible particles. This is provided in Box 1220. The density of thecompressible particles should be selected to provide for buoyancy withinthe carrier fluid. In one embodiment, each of the compressible particleshas a density that is between 12.0 ppg and 12.8 ppg, inclusive.

The method 1200 also includes selecting a geometry for the compressibleparticles. This is indicated at Box 1230. In one aspect, the selectedgeometry is either a circular profile or an oval profile. In anotheraspect, the selected geometry is polygonal. The compressible particlesmay share the same polygonal geometry or they may have random geometriesthat vary among them.

The method 1200 additionally includes selecting a compressibilityresponse (or “compressibility”) for the compressible particles. This isprovided in Box 1240. The selected compressibility response is between10% and 30%. More preferably, the compressibility is between 14% and27%, inclusive, up to 10,000 psi.

The method 1200 further comprises determining a diameter of weightingagent particles residing within the annular region. This is seen in Box1250. The weighting agent particles represent particles of barite orhematite or other residual agents from a drilling mud.

The method 1200 then includes selecting a diameter for at least aportion of the compressible particles that is at least 100 times greaterthan the determined diameter for the weighting agent particles. Thisstep is shown in Box 1260, and provides enlarged particles for the fluidmixture.

The method 1200 also includes adjusting the density of the compressibleparticles to ensure buoyancy in the presence of the weighting agentparticles. This is indicated in Box 1270. In this respect, once adiameter is provided for the enlarged portion of the particles, densitymay need to be reduced in order to preserve buoyancy.

In the method 1200, each of the compressible particles may have an outerdiameter that is between 10 μm and 700 μm (in dry state). Morepreferably, the particle size distribution will be between 40 μm to 700μm. Together, the particles may each have a porosity of between 5% and40%, or the particles together may have an average porosity of between10% and 25%.

Each of the compressible particles is fabricated to collapse in responseto fluid pressure within a confined volume. Each of the particlespreferably comprises carbon. At least some of the compressible particlesmay comprise graphite or graphene beads.

In one aspect, the compressible particles comprise first compressibleparticles having a first selected density, and second compressibleparticles having a second selected density. The first density is greaterthan the second density.

The method 1200 may optionally also include applying an elastomericcoating to the compressible particles. This is provided in Box 1280. Theelastomeric coating may comprise neoprene, polyurethane rubber, vinyl,nitrile rubber, butyl rubber, EPDM rubber, silicone rubber, orcombinations thereof. In one aspect, the elastomeric coating of each ofthe compressible particles has a negative electrical or magnetic charge,causing the compressible particles to repel each other.

The method 1200 may also include mixing compressible particles into thecarrier fluid, forming a fluid mixture. This is shown at Box 1290. Inone aspect, the compressible particles are at a concentration of 5% to40% of the fluid mixture, by volume.

A method of placing particles within a wellbore is provided. Theparticles are designed to volumetrically compress as pressure increasesin a confined volume such as a trapped annulus, thereby mitigatingdetrimental effects due to latent pressure increases upon the wellboretubulars forming the annulus. For example, as a well begins producinghot fluids from deep production zones, the hot fluids may increase thepressure within the tubing, which in turn can heat the casingsurrounding the tubing. These temperature increases may result inincreases in annular pressure. While land-based wellheads can providemeans for bleeding off the pressure increases, subsea wellheadstypically do not provide means for bleeding pressure increases. Theconsequence of this can be collapse or burst of wellbore tubulars. Themitigating compressibility and resilience of the particles serves toprevent the pressure from increasing as much as it otherwise wouldwithout the compressible particles being placed in the annulus. Thecompressibility results in additional compressible volume into which theannular fluid can expand, at pressures below the damage point of thetubulars.

A useful method for mitigating pressure increases within a wellbore isprovided that includes placing compressible particles within an annulararea of a wellbore that includes a first string of casing positionedwithin an upper portion (meaning, closer to the well surface) of thewellbore and a second string of casing extending within at least aportion of the first string of casing and into an extended portion(meaning further downhole with respect to the surface region of thewellbore) of the wellbore below (meaning further downhole with respectto the surface region) the first string of casing. The first string ofcasing surrounds an overlapped portion of the second string of casing,thereby creating a fluid-filled annulus between first string of casingand the overlapped portion of the second string of casing. The annularvolume may also include the volume between the second string of casingand the formation adjacent the extended portion of the wellbore.

The concentration of the compressible particles are dispersed in a fluidmixture that comprises a carrier fluid (either an aqueous or hydrocarbonbased liquid that does not promote premature degradation of theparticulates) and a concentration of compressible particles dispersed inthe carrier fluid. The “concentration” generally refers to a determinedweight or volume of the particulates within the carrier fluid, but doesnot require a specifically determined concentration value. A pluralityor dispersion of particles is also suitable. The carrier fluid mayinclude other components such as corrosion inhibitors, gel, viscosifyingagents, etc. The concentration of the compressible particles may bedetermined by volume or weight, and will vary depending upon thecompressibility of the particles, the pressures anticipated within thewellbore, expected tubular volume change, permissible tubular pressurerange, and projected annular volume expansion. Exemplary concentrationsof mineral-based particulates, such as graphitic or carbon basedcompositions, without limitation, may be for example, from 1 pound perbarrel (ppb) of carrier fluid to 125 ppb, or 1 ppb to 100 ppb, or 1 ppbto 50 ppb, or 1 ppb to 25 ppb, or 1 ppb to 10 ppb. Suitableconcentrations of polymeric compositions may be proportionately lowerwith respect to their lower bulk density or specific gravity as comparedto the mineral-based particulates. Lab testing under projected annularconditions is recommended to ascertain the desired particulateconcentration and performance factors. Some wellbore annular tubularcombinations and/or production temperature conditions may requiresubstantial protection from pressure increases and correspondingsubstantial deformation and/or resiliency, while others may only requirea nominal amount of protection.

The concentration may be considered a plurality of particles, as thedesired particles should remain individually granular particulates whensubjected to pressures of up to 10,000 psig, without packing or losinggranularity. It is preferred that the particles remain as distinctparticles over when indefinitely subjected to pressure of up to 10,000psig at maximum expected wellbore temperature. Resiliency is the abilityof the particulate particles to absorb energy by volumetricallycompressing, and then upon release of the energy to recover a percentageof the lost volume. The claimed particles must not only compress underincreasing pressure, but they also must be able to recover at least 50%of the compressed volume upon release of that pressure. The volumerecovery is the resiliency. It is the volume recovery that is importantin the annulus fluid mixture, because over its life, a trapped annuluswill experience numerous pressure (and thermal) changes due to changesin production activity, shut-in periods, stimulation, well treatments,etc. Particulates that merely compress without sufficient recovery(resilience or elastic expansion) are not useful for pressurecompensation purpose over the life of the well. An ideal recovery wouldbe 100% by volume.

What is suitable for the present technology is a compressibleparticulate concentration that, after initial placement in the wellbore,can volumetrically compress or deform (e.g., compress, elongate,flatten, pack together, shrink in bulk diameter, and/or otherwise changein bulk volume for the plurality of particulates) by at least 10% byvolume, or at least 20%, or at least 50% in response to furtherincreased applied pressure after placement in the annulus, that is, ascompared to their volume in the annulus immediately after placementtherein, even if them somewhat compressed. Stated differently, althoughthe compressible particles may compress somewhat in response to theirinitial pumping and placement in the wellbore, such as in conjunctionwith a cementing job, wherein they experience an initial hydrostaticpressure profile within the annulus, it is the further or subsequentcompression that is relevant for the particles performing their intendedfunction in the annulus. That further or subsequent compression may bedue to any of a number of operational factors such as thermal heatingdue to production operations. Thereafter, the particles should be ableto recover at least 50% of that further, or subsequent compression(deformation).

The important compressibility and resilience feature that is key is theability to further compress after initial placement in the wellbore.Initial placement in the wellbore refer to the time when the particlesare first pumped into the wellbore annulus and pumping is ceased suchthat the fluid mixture with the particles is generally at its intendedlocation, such as at the end of pumping cement and displacing the wiperplug, before cement sets up. The further important function of theparticles is their ability to then resiliently recover from thatadditional deformation or compression that happened subsequent toinitial annular placement. The particles should be able to furthercompress by at least 10% by volume, or at least 20%, or at least 50% inresponse to the subsequently applied pressure (subsequent to initialplacement in the annulus). This subsequent or further pressure increasemay be caused by wellbore activities or events as described previouslyherein, such as production of hot produced fluids through the wellbore.

Thereafter, when the applied fluid pressure is removed or reduced, theparticles resiliently recover and reform or expand back volumetricallyby at least 50%, or at least 60%, or at least 70%, or at least 100%, oreven at least 110%, with respect to the volume of the particulates inthe annulus immediately after placement, prior to the subsequentapplication of the pressure in the annulus. Preferred particles will beable to effectively repeat this cycle many times. The ability of theparticles to reform by at least 50% is desirable, more than 60% moredesirable, and by at least 80% even more desirable. Recovery of 100% isideal, but many particulate materials will likely experience at leastsome mild degree of hysteresis, such as some fluid permeation into a fewof the pores of some portions of some of the particulates. The featureof recovery or expanding back after removal of the pressure increase ispreferably an elastic strain recovery. The concept of recovery isreferred to herein as resilience.

Thereby, the compressible particulates are useful for compensating forsubsequent, repeated pressure increases according to their intendedpurpose, over a desired life-range in the wellbore annulus. From astress-strain perspective, it is desirable that the volume changes dueto pressure increases of up to 10,000 psig can be repeatedly applied,even if not to the same degree as for previous volume changes. Thepreferred particles exhibit at least 10% of resiliency, by volume. Asfor particulate dispersion or distribution within the annulus, it is notrequired that the particulates remain evenly or otherwise distributedover the length of the wellbore annulus. The particulates may, overtime, accumulate in a lower region of the annular area, due togravitational deposition. So long as the desired, overallcompressibility and recovery or resilience of the plurality ofparticulates provides the desired amount of resiliency or recovery, thenaccumulation in the lower portion of the wellbore should not bedetrimental. However, if the accumulation in the lower portion of thewellbore becomes excessive to the point that the particulates within thelower portion of the packed volume do not experience the pressureincrease, then care should be considered to determine whether thecarrier fluid should have some gel or particulate-suspending propertiesthat will keep the particulates suspended in a fashion that will permitsufficient volume of the particulates to pressure exposure andcontribute to the desired pressure compensation in the annulus.

In another exemplary embodiment, the compressible particles maycomprise: first compressible particles having a first degree ofcompressibility; and second compressible particles having a seconddegree of compressibility; and wherein the first degree ofcompressibility is higher than the second degree of compressibility.

In yet another exemplary embodiment, the compressible particles maycomprise: first compressible particles having a first density; andsecond compressible particles having a second density; and wherein thefirst density is greater than the second density.

INDUSTRIAL APPLICABILITY

The systems and methods disclosed herein are applicable to the oil andgas industries.

It is believed that the disclosure set forth above encompasses multipledistinct inventions with independent utility. While each of theseinventions has been disclosed in its preferred form, the specificembodiments thereof as disclosed and illustrated herein are not to beconsidered in a limiting sense as numerous variations are possible. Thesubject matter of the inventions includes all novel and non-obviouscombinations and subcombinations of the various elements, features,functions, and/or properties disclosed herein. Similarly, where theclaims recite “a” or “a first” element or the equivalent thereof, suchclaims should be understood to include incorporation of one or more suchelements, neither requiring nor excluding two or more such elements.

It is believed that the following claims particularly point out certaincombinations and subcombinations that are directed to one of thedisclosed inventions and are novel and non-obvious. Inventions embodiedin other combinations and subcombinations of features, functions,elements, and/or properties may be claimed through amendment of thepresent claims or presentation of new claims in this or a relatedapplication. Such amended or new claims, whether they are directed to adifferent invention or directed to the same invention, whetherdifferent, broader, narrower, or equal in scope to the original claims,are also regarded as included within the subject matter of theinventions of the present disclosure.

What is claimed is:
 1. A method of designing compressible particles for a fluid mixture, wherein each of the compressible particles is fabricated to collapse in response to fluid pressure within an annular region of a wellbore, comprising: determining a specific gravity of an aqueous carrier fluid; selecting a desired depth for the compressible particles within the annular region; selecting properties for the compressible particles to provide for buoyancy of the compressible particles within the carrier fluid such that the compressible particles are designed to stay suspended more densely at the desired depth within the annular region, and wherein the properties comprise: a variety of specific gravities within a range between plus/minus 0.5, inclusive, of the specific gravity of the carrier fluid; a porosity in a range between 5% and 40%; a variety of irregular particle shapes; and a variety of outer diameters in a range between 10 μm and 700 μm (in dry state); and selecting a compressibility response for the compressible particles, wherein the compressibility response is optimized for the desired depth.
 2. The method of claim 1, wherein: selecting the desired depth comprises identifying a lowest depth and a highest depth for placement of the compressible particles; and the method further comprises determining an initial pressure state (P_(a)) of the annular region and a final pressure state (P_(b)) of the annular region at the desired depth before selecting the compressibility response.
 3. The method of claim 2, wherein: the compressible particles comprise first particles having a first compressibility response, and second particles having a second compressibility response, wherein the first compressibility response is higher than the second compressibility response; and the method further comprises placing the first particles proximate the highest depth and placing the second particles proximate the lowest depth.
 4. The method of claim 3, wherein the first particles have a density that is less than a density of the second particles.
 5. The method of claim 1, wherein: each of the compressible particles comprises carbon; and the selected compressibility response of the compressible particles is between 10% and 30%, up to 10,000 psi.
 6. The method of claim 5, wherein each of the compressible particles has a resiliency that is in a range between 80% and 120%, inclusive.
 7. The method of claim 6, wherein the compressible particles have an average porosity of between 10% and 25%.
 8. The method of claim 6, wherein each of the compressible particles comprises an elastomeric coating that serves as a wetting agent to enhance particle disbursement. 